In telecommunications systems, transmission facilities are known to include either loaded or nonloaded two-wire bidirectional cable. For two-wire cables having lengths greater than a prescribed minimum, it is common practice to employ signal repeaters. In so doing, it is typical to connect the two-wire bidirectional cable through a transmission network to unidirectional receive and transmit paths or facilities. The transmission network includes circuit arrangements for maximizing signal loss between the receive and transmit paths, thereby minimizing unwanted signal reflections, commonly referred to as echoes. When a loaded, two-wire cable is encountered, a first type circuit arrangement is employed tailored to the impedance characteristic of the loaded cable. Similarly, when a nonloaded cable is encountered, a second type circuit arrangement is utilized tailored to the impedance characteristic of the nonloaded cable.
Heretofore, office records were relied upon to identify whether the two-wire facility was either the loaded type or the nonloaded type. Such practice is somewhat satisfactory for repeater arrangements, which are manually connected and adjusted. Unfortunately, office records are often inaccurate or out of date; consequently, installation errors may occur, resulting in less than desirable transmission performance.
More recently, two-wire facilities have been identified as to type by employing an electronic measurement arrangement. The electronic measurement arrangement disclosed in U.S. patent application, Ser. No. 19,368, filed Mar. 12, 1979, by J. L. Neigh and R. G. Sparber, now U.S. Pat. No. 4,224,483, issued Sept 23, 1980, employs an impedance measuring circuit which is connected directly to the two-wire bidirectional facility. The measuring circuit includes a complex excitation signal generator and filter arrangement to form a feedback loop which oscillates at a given frequency. Phase relations imposed by the feedback loop resuult in the magnitude of the filter output signal being proportional to the real part of the two-wire facility load, which is driven by the excitation signal. Additionally, the circuit operation requires connection of a reference load and a capacitor charging circuit to establish a reference level to which the measured value is compared for determining whether the two-wire facility is a loaded or nonloaded type.
Connection of such arrangements directly to two-wire facilities typically requires the use of relays, or the like, which can withstand surge voltages sometimes found on subscriber facilities. Use of relays is undesirable because of their cost and space requirements. Moreover, switching of such arrangements onto the bidirectional facility creates undesirable noise. Thus, although this prior identification arrangement may be advantageously employed in some applications, it is undesirable for use in others, especially where space requirements dictate elimination of the relays and use of less complex circuit arrangements.